Method of magnetometric detection of explosive objects.

Method of magnetometric detection of explosive objects.

ZVEZHINSKY Stanislav Sigismundovich, Doctor of Technical Sciences
PARFENTSEV Igor Valerievich, Candidate of Technical Sciences

METHOD OF MAGNETOMETRIC DETECTION OF EXPLOSIVE OBJECTS

Source: Magazine «Special Equipment and Communications»

The problem of humanitarian demining, as well as the search for unexploded ordnance (UXO) are currently very pressing. On the territory of more than 70 «problem» countries, from 60 to 120 million mines have been laid (according to various sources), not to mention the existence of millions of unexploded shells and aerial bombs left not only from the Second World War, but also as «remnants» of military testing grounds (there are more than 20 of them in the USA alone). Every year, about 26 thousand people die from mines in the world, in countries such as Angola, on average, one in 334 has a limb amputated, in Cambodia there are more than 25 thousand such disabled people, the number of mines exceeds the number of inhabitants. Other problematic countries are Afghanistan, Iraq, Kuwait, India, Colombia, Lebanon, Yemen, Mozambique, Chad, Nepal, Bosnia, etc. More than 22 million people in the world are exposed to mine risk every day, and the crisis associated with this factor is predicted to only grow [1, 2]. Two thirds of the countries have joined the Convention (which came into force on March 1, 1999) on the prohibition of anti-personnel mines — the most common explosive ordnance (EOR). But Russia, the USA, Israel, China — the world's largest producers — were not among them (as of early 2005).

There are many organizations in the world, in addition to military engineering units, that solve practical problems of searching for explosive remnants of war and demining areas; among them, the Geneva International Centre for Humanitarian Demining (GICID) occupies a key place [3]. Periodic international scientific and practical conferences and workshops are held (the most significant are UXO Forum, UNMAS Conf., US HDR Workshop, NDR Forum, etc.), several research laboratories at universities (Canada), applied research institutes (Germany, Great Britain) or military research laboratories (USA) conduct research to increase the efficiency of search. However, due to the complex nature and complexity of the problem, there is no single optimal way to detect and identify explosive remnants of war [2, 3].

Chemical (smell) and biological (dogs, rats and even insects) search methods, although used, are obviously subjective, and therefore unreliable. Physical methods of detecting explosive hazards are predominantly used: active electromagnetic probing of the surface soil layer with electromagnetic pulses and sinusoidal fields (metal detectors 2 — 50 kHz, ground penetrating radar 100 — 900 MHz), seismic waves and neutron radiation, recording anomalies in electrical conductivity and soil density, measuring infrared and gravitational fields, etc. [2 — 5]. Each has its own advantages and disadvantages, it is impossible to single out the optimal one, otherwise the industry would produce it, and engineers would use only it.

The most widely used are harmonic (FM – frequency domain) or pulse (TD – time domain) active metal detectors or metal detectors, the operating principle of which is based on the registration of the secondary electromagnetic field induced by Foucault currents in a metal body under the action of the excited primary field [3, 5]. Currently, more than 30 companies abroad and in Russia produce such devices, the most famous are CEIA (products MIL-D1, MIL-D1/DS, Italy), Vallon (VMC1, VMH2, VMH3, VMH3CS, VMM3, VMW1, Germany), Ebinger (EBEX-420, EBEX-535, Germany), Fisher (1235-X, 1266-XB, Germany),Minelab Electronics (FIA4, F3, F1A4, Australia), Shiebel Electronics (AN-19/2, ATMID, MIMID, Austria), Geonics (EM61-Mk2, Canada), Guartel (MD4 , MD8, MD2000, UK), Whites (AF-108, DI-PRO-5900, MXT-300, DFX-300, Spectrum-XLT, USA), Garrett (GTAx-550, GTP-1350, GTI-2500, USA), “AKA-Control” (“Pilgrim-7246”, «Condor-7252», «Vector-7262», Russia).

Among the methods of detecting explosive devices, a significant place is occupied by the search for magnetic anomalies (MAD), which are created by ferromagnetic metal shells of the absolute majority of explosive devices [2, 4, 6]. At the same time, shell-less explosive devices or special ammunition are not detected, but they have the least destructive power, and are also unstable to detect by other methods. MAD is one of the «deepest» search methods, allowing the detection of explosive devices (for example, large aerial bombs, land mines) at depths of up to 8 m. In addition, the magnetometric method is passive, which ensures that the explosive device is not detonated by initiating physical fields during active probing, which is often necessary.

In the literature, the method of searching for VOPs based on the detected anomalies of the Earth's magnetic field (EMF) is singled out as one of the most promising [2, 7 – 9], the achievable characteristics of recording devices – magnetometers and gradiometers – are substantiated, and limitations are shown. In this paper, some provisions of the magnetometric method of searching for VOPs are developed.

The method is implemented using passive «vector» gradiometers, which eliminate to the maximum extent the effect of the constant (main) magnetic field, which has an insignificant spatial gradient. Such devices use two identical sensors — ferroprobes [10], spaced along the sensitivity axis (SA) by 25 — 170 cm and registering magnetic anomalies with a large gradient, possibly associated with the VOP [6, 9]. Non-gradiometric search devices, usually based on quantum magnetometers with optical pumping of cesium or potassium vapor (Geometrix G-858, USA; Scintrex NAVIMAG, UK) are used mainly to remove a map of the magnetic field on the ground. After this, it is possible to search for the VOP using the map of magnetic anomalies, which is difficult «on the fly». In addition, quantum-optical devices are more likely to be classified as scientific devices and have, even in relation to expensive flux-gate gradiometers, an increased cost (about 20 thousand dollars), and require more careful and competent handling than is prescribed for conventional search devices.

There are approximately 3 times fewer manufacturers of magnetometric devices for searching for explosive remnants of war in the world than manufacturers of metal detectors, and in Russia there is only one — Research Institute «Project», Tomsk (product «MBI-P»). Abroad, these are, first of all, Institute Dr.Forster or Foerster (FEREX 4.032, Germany), Ebinger (MAGNEX 120LW, Germany), Vallon (EL1302D2, Germany), Schontedt Instrument (GA-72 Cd, GA-52Cx, GA-92XT, USA), CST (Magna-Trak, USA), Geoscan Research (FM-256, UK), Bartington Instruments (Grad601, UK). The greater prevalence of metal detectors is due to three main reasons:

1. significantly lower cost;
2. the ability to detect any metals;
3. extended scope of application – search for treasures, pipelines and cables in the covering layer, archeology.

Available scientific papers and reports indicate that the best characteristics for detecting explosive ordnance are those devices that combine passive magnetometric and active electromagnetic detection principles, such as ERDC EM61HH & G-822, SAIC STOLS/VSEMS (on a bicycle base), SAIC MSEMS [11–13]. Such systems, which detect any type of metal, are typically designed as a series of sensors. They are designed on a bicycle or automobile base, are very expensive, and are manufactured in individual units. Table 1 presents comparative characteristics of passive magnetometers and active metal detectors, analyzed on the basis of a number of papers [1, 2, 4, 7, 11–14] and obtained through expert assessments.

Table 1. Comparative characteristics of passive gradiometers and active metal detectors for searching for explosive ordnance

Thus, the advantages of passive gradiometers over metal detectors are:

1. on average 2 times greater (compared to metal detectors) maximum depth of explosive ordnance search in steel (ferromagnetic) shells;
2. independence of operation from soil conductivity, climatic conditions, presence of water;
3. high accuracy of target localization and potential ability to reliably predict the depth, type and orientation of explosive ordnance in space;
4. the possibility of combining into a multi-sensor system (portable or on a wheeled base), providing the highest possible search speed «on the go».

For an active metal detector, it is not so much the mass of a specific explosive device that is important, but the surface area associated with its diameter d. At the same time, as practice shows, for assessing the maximum depth hMAX detection of explosive ordnance using a “good” active metal detector in dry soil conditions (conductivity 104 − 105 Ohm×m), the engineering formula [11] is acceptable:

h1MAX ≈ 11d, (1)

where d is the diameter (minimum dimensions) of the explosive ordnance.

The useful qualities of the explosive ordnance as an object of passive magnetometric detection are determined by: 1) mainly the mass m (volume V) a ferromagnetic protective shell, which is usually not less than the mass of the explosive (HE); 2) to a lesser extent, the shape of the object, characterized by the ratio of the maximum geometric size to the minimum or the ratio of length to diameter; 3) to the least extent, the magnetic permeability μ of the ferromagnet.

In this case, the maximum search depth hMAX is associated with the achievable sensitivity of the gradiometer dB/dr, as well as with the shape and mass of the ferromagnetic shell by a complex relationship. It is simplified if we assume that: 1) the shape of the «soft magnetic» HEP is a ball; 2) magnetic permeability μ≥ 100 (typical); 3) the location of the gradiometer OC and the magnetic moment M of the ball acquired in a constant magnetic field with induction BT is the best, coaxial; 4) magnetic noise and interference are much less than the sensitivity of the gradiometer. Such a formula is given in [8], taking into account other variables it is reduced to the form:

, (2)

where [dB/dr] = nT/m − achievable sensitivity, [BT ] = nT.

On the territory of the Russian Federation, the magnetic inclination varies from almost 90° (at high latitudes, beyond the Arctic Circle) to 57° (Vladivostok). The value of ВТ at the middle Russian latitudes (St. Petersburg − Astrakhan) can be estimated based on known data [21]: ВТ ≈ (5.4 ± 0.4) × 104 nT. At the same time, the vertical component BВ of the main magnetic field on the territory of the Russian Federation is on average 2.8 times higher than the horizontal BГ and is dominant, on average it can be assumed: BГ≈ 18 μT; BВ ≈ 50 μT.

The best products – gradiometers (such as FEREX 4.032, VALLON EL1302 D2) are characterized by their own noise at a level of ~ 0.3 nT, which on a base of 0.5 – 0.65 m gives an estimate of the threshold sensitivity of ~ 0.5 nT/m [16, 17]. However, such sensitivity cannot be realized in real conditions – the noise of the magnetic field and the “non-ideality” of the gradiometer – errors of misalignment (magnetometric transducers) and inequality of conversion coefficients – interfere. As shown in [6, 8, 18, 20, 22], for a typical environment (equivalent noise no more than 2 – 3 nT), it is possible to achieve a detection threshold of (dB/dr)MIN = 10 nT/m.

Then, when substituting BT and (dB/dr)MIN into (2), we obtain an estimate of the maximum depth of detection of the VOP by the gradiometer:

h ≈ 8 d3/4. (3)

By equating (1) and (3), we can conclude that for real VOPs with a diameter of less than 30 cm, the use of a gradiometer gives better results. In wet soil conditions (conductivity 102 − 103 Ohm×m), in the presence of constant magnetization and «elongation» of a real object, a large maximum detection range for a gradiometer is almost always ensured.

An experimental comparative analysis of the detectability of metal detectors and gradiometers confirms (3) and shows that for “small” and “medium” explosive devices (caliber from 20 to 81 mm) at search depths of up to 0.5 m (and typical soils), the former are better [11, 23]. In the range of explosive device calibers of 100–155 mm, the characteristics are comparable; further, the gradiometer has an advantage. However, if the dimensions of the metal detector’s transmitting/receiving antenna are relatively large (Vallon VMH 3CS, diameter ~1 m), then the probability of detecting explosive devices by a metal detector at depths of up to 1.5 m is even somewhat higher than that of the passive FEREX 4.032 [121]. Thus, metal detectors have a higher detectability of explosive devices at those relatively small depths (up to 1 m) where they function stably.

In Table 2 presents the mass and size characteristics of domestically produced explosive devices and the maximum depth of penetration upon impact with loam-type soil. When a mine or landmine is installed at a depth of more than 1 m, its effect is sharply weakened. Table 3 presents the characteristics of typical NATO explosive devices according to data from [11, 15].

Table 2. Mass and size characteristics of explosive objects

Table 3. Characteristics explosive ordnance from NATO countries

 In manual gradiometers, two identical magnetometric transducers (MT) − ferroprobes are placed in the measuring module or probe on a “rigid” base of length a, their sensitivity axes are parallel to the base [7, 8]. In most known products a= 0.25¼1 m, and quite recently (3-4 years ago) products with a = 1.6¼1.7 m (Foerster, Vallon) appeared [14, 16, 17]. However, the use of the latter does not imply a search “on the fly” with a typical speed of 0.2 – 1 m/s, but rather a refinement of the location of the VOP in a practically stationary mode.

Fig. 1 shows a conventional scheme for measuring the magnetic induction anomaly using a gradiometer with a base a; height ∆ lower MP1 above the surface (5 – 10 cm) compared to the probable depth h of the VOP is assumed to be small.

Fig. 1. Measuring scheme for searching for VOP using a gradiometer

The conventional coordinate axes X, Y, Z can be associated with the direction along the meridian, latitude and radius to the center of the Earth, respectively, the coordinates x, y can be tied to another convenient measurement grid. In this case, the direction of movement when searching for the VOP occurs conventionally along the OX axis at equal intervals (typically 1 m), laid off along the OY axis. The vector of the MPZ BMPZ is directed toward the center of the Earth (in the southern hemisphere, on the contrary, from the center) at an angle with an inclination of j.

At a depth of h in the thickness of the soil there is a possible detection object — a VOP in a ferromagnetic shell with induced (in the MPZ) and/or residual magnetization. The latter is a random value acquired mainly during the manufacture of the VOP (heat treatment), and is not subject to predictable assessment — even for similar serial ferromagnetic objects it varies within more than 20 dB. As shown in the works [15, 18, 19], when an artillery or aircraft VOP hits the ground, its almost complete impact demagnetization or loss of residual magnetization occurs («shaking» of domains). Induced magnetization depends on the magnetic permeability of the ferromagnet and its shape and can be estimated with an error of about ±3 dB. Usually, only it is taken into account in the estimated calculations of the detectivity of the gradiometer.

However, this is not entirely true for mines, where residual magnetization may prevail. Anti-tank (anti-vehicle) mines are usually made in the form of a cylinder or parallelepiped with the largest size (diameter) of 15 — 30 cm, thickness from 5 to 9 cm. They are laid at various depths of at least 15 cm. Anti-personnel mines are made in the form of disks or cylinders with a diameter of 2 — 13 cm, length of 5 — 10 cm, and can weigh less than 30 g. They are installed on the surface of the earth or at a depth of no more than 5 cm (at greater depths, their destructive ability decreases).

Alternating field B(t), which is recorded by the gradiometer during the search, is interference. The field is generated by geomagnetic fluctuations (including magnetic storms and substorms, geomagnetic noise), fields from industrial currents, − as a rule, the main frequency of the industrial network f = 50 Hz (in the USA — 60 Hz) and its harmonics [21]. However, the main indirect cause of the appearance of an interference signal when monitoring VOP is the action of the EMF due to errors in the measuring part of the gradiometer associated with differences in two MP:

1. imbalance ∆G conversion coefficients;
2. misalignment (divergence) ∆φ of their sensitivity axes.

The most difficult to minimize manufacturing error is the second one, during operation, shaking, unintentional impacts, temperature changes contribute to a chaotic increase in ∆φ. Various methods are used for compensation, divided into two groups — electrical and mechanical. The maximum achievable mechanical value of misalignment is achieved in modern products (for example, Institute Dr. Forster) ∆φ ≈ 0.01° [17]. With the initial position of the gradiometer OC perpendicular to the field lines of the EMF, this misalignment during monitoring leads to the appearance of an interference signal of the order of Bpom ≈ BT×∆φ ≈ 9 nT.

The EMF is not completely uniform — there is a gradient on the earth's surface, but it is negligibly small and does not limit the potential sensitivity of the gradiometric method for detecting EMF. Gradient dВ/dr of the vertical (Z) component of the main EMF at any point on the earth's surface does not exceed 0.03 nT/m − at the poles, on the territory of the Russian Federation it is less by ~6 dB, at the equator it is zero [21, 25]. At the same time, on a base of a ≤ 1 m, such spatial non-uniformity of the EMF can lead to a maximum difference error signal ∆Вer ≤ 0.03 nT, which is at the level of the intrinsic noise of modern ferroprobes, and can be neglected.

Urban magnetic noise caused by the superposition of fields from various industrial sources, as practice shows, reaches:

Vshgor @ 10…100/ a, nT /m. (4)

Therefore, the gradient of industrial interference near sources of strong currents (electrified transport, electric railways, high-voltage power lines, etc.) may exceed the imbalance error of the gradiometer. As a result, the products are equipped with an adjustment that reduces sensitivity in places where the noise level is higher than usual, which reduces the depth of the VOP search.

A generally applicable magnetic model of a VOP with a ferromagnetic volume V is a magnetic dipole with a moment M, the value of which is determined by the vector sum of the induced magnetization Jу and the residual magnetization Jо: M = (Jу+Jо)×V.The induced magnetization depends on the shape of the ferromagnet and is determined precisely only in the case of an isotropic ellipsoid:

Jand = | | c || HT, (5)

where || c || is a symmetric shape susceptibility tensor consisting of three coefficients {cx, cy, cz}, where: ci = c/(1+c × N i ) , i = x , y , z are the indices of the symmetry axes of the object and the corresponding coordinate system; c = (μ−1) is the susceptibility of the ferromagnet; Ni − demagnetization coefficients along the corresponding axes, related by the normalization condition S Ni = 1 (for a sphere Ni = 1/3), depending on the ratio of the axis lengths [21, 25].